The present invention relates generally to tubular air heater constructions used in steam generator installations for utility power generation and, more particularly, to an air heater gas inlet plenum for such tubular air heaters which permits retrofit installation of additional flue gas environmental treatment equipment.
Increasingly stringent environmental regulations continue to place pressures upon electric utilities which utilize fossil-fueled steam generators to produce electricity. However, modifications to existing steam generators is often problematic due to the limited space available, and the utilities"" desire to make such modifications in an efficient manner and at minimum cost.
Many fossil-fueled steam generators used for utility power generation employ recuperative tubular air heaters to transfer heat from the hot flue gases produced during combustion into the incoming combustion air. After this heat exchange process has occurred, the flue gases are eventually conveyed into the atmosphere via a stack. Various types of environmental cleanup equipment are also often provided in between the flue gas outlet of such steam generators and the stack, such as precipitators for particulate control and wet or dry flue gas desulfurization systems.
In addition to controlling sulphur oxide emissions, nitrogen oxides are one of the primary pollutants emitted during combustion processes. As described in Chapter 34 of Steam/ its generation and use, 40th Edition, Stultz and Kitto, editors, Copyright(copyright)1992 The Babcock and Wilcox Company, at pages 34-1 to 34-9, various methods are used to control nitrogen oxide emissions. One method involves selective catalytic reduction (SCR) systems which catalytically reduce flue gas NOx to N2 and H2O using ammonia in a chemical reduction. For further details concerning the basic principles behind SCR systems, the reader is referred to the aforementioned Chapter 34 of Steam, the text of which is hereby incorporated by reference as though fully set forth herein.
SCR reactions take place within an optimal temperature range, and the SCR modules used in such systems are typically located downstream of the economizer gas outlet of the steam generator or boiler and upstream (with respect to a direction of flue gas flow through the steam generator) of the air heater devices used to preheat the incoming combustion air. Retrofit applications of SCR systems to steam generators having tubular air heaters present their own particular problems.
Referring generally to FIGS. 1 and 2 of the present disclosure, there are shown two such typical installations of tubular air heaters as provided at the economizer gas outlet of a utility steam generator (not shown).
FIG. 1 illustrates a first embodiment of a known tubular air heater installation, generally designated 10. Hot flue gas 12 provided from a boiler economizer gas outlet 14 is conveyed to a tubular air heater 16 for transferring heat from the hot flue gases 12 into incoming combustion air (not shown). Tubular air heater 16 is generally provided with a large gas inlet plenum 18 located at an upper end of the tubular air heater 16 and above a plurality of heat exchanger tubes 20 for receiving and conveying the hot flue gas 12 therethrough while the air for combustion (not shown) passes there across over the outside surfaces of these tubes 20 in a heat exchange relationship. The large gas inlet plenum 18 has a top wall 19, a rear wall 21, and side walls 23.
The hot flue gases 12 are cooled as they pass downwardly through the tubular air heater 16, conveying the heat contained therein to the combustion air. A gas outlet plenum 22 is generally provided at the bottom of the tubular air heater 16 for collecting and conveying the flue gases 12 to a stack (not shown). Since the flue gases 12 contain significant amounts of flyash, a hopper 24 may be provided at a lower portion of the gas outlet plenum to collect flyash particles. As illustrated in the arrangement of FIG. 1, the hot flue gas 12 is conveyed from the boiler economizer gas outlet 14 via an outlet gas flue 26 into the gas inlet plenum 18 where the flue gases make a 90xc2x0 turn downwardly into the plurality of heat exchanger tubes 20 and, after exiting therefrom, make another 90xc2x0 turn at the gas outlet plenum 22 before exiting the tubular air heater 16. The tubular air heater 16 is generally top supported via support rods 28 which are hung from top support steel 30, but it may be bottom supported as well.
Various other arrangements of tubular air heaters are known. For particular details of such arrangements, the reader is referred to Chapter 13 of Steam/ its generation and use, 39th edition, Copyright(copyright)1978 by The Babcock and Wilcox Company, particularly pages 13-4 to 13-8, and that chapter is hereby incorporated by reference as though fully set forth herein. FIG. 2 illustrates one such variation, generally designated 50. Throughout FIGS. 1 and 2, as well as in the balance of the present disclosure, like reference numerals designate the same or functionally similar elements throughout the several drawings. In the arrangement of FIG. 2, the hot flue gases 12 are again conveyed from the boiler economizer gas outlet 14 into an upper plenum 18 of a U-shaped tubular air heater, generally designated 17. As shown therein, tubular air heater 17 is provided with a gas downflow section 32 and a gas upflow section 34 containing heat exchanger tubes 20. While the hot flue gas 12 makes a 90xc2x0 turn downward into the gas downflow section 32, at the bottom portion thereof the hot flue gases 12 make a 180xc2x0 turn up into the gas upflow section 34 by means of an intermediate gas plenum 36. If desired, hoppers 24 may again be provided to collect particles from the hot flue gas 12 as it makes the 180xc2x0 turn within the intermediate gas plenum 36. After passing upwardly through the heat exchanger tubes 20 comprising the gas upflow section 34, the hot flue gases 12 make another 90xc2x0 turn in a gas outlet plenum 38 located at an upper portion of the gas upflow section 34. The hot flue gases 12 then exit from the tubular air heater 17 for eventual exhaust into the atmosphere. At the upper portion of the tubular air heater 17 a flue gas impermeable wall 40 separates the gas inlet plenum 18 from the gas outlet plenum 38. Similarly, flue gases conveyed downwardly through gas downflow section 32 and upwardly through gas upflow section 34 are separated from one another by another flue gas impermeable wall 44.
It will thus be readily appreciated that retrofit installation of SCR equipment so that the flue gas 12 exiting from the boiler economizer gas outlet 14 can be treated prior to entry into the tubular air heater configurations illustrated in FIGS. 1 and 2 can present significant arrangement problems. The present invention provides a solution with a minimum of cost and modification.
The present invention involves modifications to an air heater gas inlet plenum for a tubular air heater to permit retrofit installation of additional flue gas environmental treatment equipment, such as selective catalytic reduction (SCR) systems for treating flue gas. A divider plate is located within the inlet plenum to subdivide it into first and second flue gas passages. The first flue gas passage created by the divider plate merely conveys the hot flue gas through the inlet plenum and into flues which convey the flue gas to the new equipment. Return flues convey the flue gas back to the second flue gas passage created in the inlet plenum which, in turn, conveys the flue gas into the tubular air heater heat exchanger tubes. By taking advantage of the large size of a conventional air heater gas inlet plenum, the. first and second flue gas passages created by the divider plate still have sufficient cross sectional area so that acceptable flue gas velocities are preserved.
Accordingly, one aspect of the present invention is drawn to a tubular air heater for transferring heat from hot flue gas into air. The tubular air heater comprises a plurality of heat exchanger tubes for conveying the hot flue gas therethrough and the air there across in heat exchange relationship. In addition, there is provided an inlet plenum means for receiving the hot flue gas, the inlet plenum means being subdivided by divider plate means therein into a first and a second flue gas passage, the first flue gas passage for conveying the hot flue gas through the inlet plenum means while preventing the hot flue gas from entering the plurality of heat exchanger tubes, the second flue gas passage for conveying the hot flue gas into the plurality of heat exchanger tubes.
Another aspect of the present invention is drawn to a gas inlet plenum for a tubular air heater having a plurality of heat exchanger tubes for conveying hot flue gas therethrough and air there across in heat exchange relationship. The gas inlet plenum comprises divider plate means therein for subdividing the gas inlet plenum into a first and a second flue gas passage. The first flue gas passage is for conveying hot flue gas from a first flue gas inlet of the gas inlet plenum through the gas inlet plenum to a flue gas outlet while preventing the hot flue gas from entering the plurality of heat exchanger tubes. The second flue gas passage is for conveying the hot flue gas from a second flue gas inlet of the gas inlet plenum into the plurality of heat exchanger tubes.
Yet another aspect of the present invention is drawn to a method of modifying a gas inlet plenum for a tubular air heater to permit retrofit installation of additional flue gas treatment equipment in between a boiler economizer gas outlet and the tubular air heater. As described above, the gas inlet plenum is fluidically connected to a plurality of heat exchanger tubes of the tubular air heater for conveying hot flue gas therethrough and air there across in heat exchange relationship, and has a first flue gas inlet, a rear wall, and side walls. The method involves modifying at least one of the rear wall and the side walls of the gas inlet plenum to provide a flue gas outlet and a second flue gas inlet therein. In addition, the method involves installing divider plate means into the gas inlet plenum for subdividing the gas inlet plenum into a first and a second flue gas passage. The first flue gas passage is for conveying hot flue gas from the first flue gas inlet of the gas inlet plenum through the gas inlet plenum to a flue gas outlet while preventing the hot flue gas from entering the plurality of heat exchanger tubes. The second flue gas passage is for conveying the hot flue gas from the second flue gas inlet of the gas inlet plenum into the plurality of heat exchanger tubes.
Yet another aspect of the above method involves modifying at least one of the rear wall and the side walls of the gas inlet plenum to provide plural flue gas outlets and plural flue gas inlets therein. The divider plate means are installed to subdivide the gas inlet plenum into a first and second flue gas passage, the first flue gas passage for conveying hot flue gas from the first flue gas inlet of the gas inlet plenum to the plural flue gas outlets while preventing the hot flue gas from entering the plurality of heat exchanger tubes, the second flue gas passage for conveying the hot flue gas from the plural second flue gas inlets of the gas inlet plenum into the plurality of heat exchanger tubes.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects obtained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.